1. Field of the Invention
The present invention relates to a method for flattening an organic film and a method for manufacturing a liquid crystal display device using the same. More particularly, the present invention relates to a method for flattening an organic film that flows a long distance with high controllability and a method for manufacturing an active matrix substrate (TFT substrate) of a liquid crystal display device using the method for flattening an organic film.
2. Description of the Related Art
In conventional technologies for flattening an uneven surface of a substrate having semiconductor devices formed thereon, there has been conceived a flattening method for coating an organic film on the uneven surface of the substrate and heating the organic film to fill concave due to the uneven surface of the substrate with the organic film to thereby flatten the uneven surface of the substrate.
Japanese Unexamined Patent Application Publication No. 7-120784 discloses an example using the above-mentioned flattening method and is explained with reference to a cross-sectional view shown in FIGS. 1A to 1C.
Referring to FIG. 1A, after a lower wiring layer 302 is formed on a glass substrate 301, polyamide acid is spin-coated on the glass substrate 301 to a film thickness of 0.1 to 1 xcexcm to cover the lower wiring layer 302. Then, the polyamide acid film is heated at temperatures of 200 to 300xc2x0 C. to be imidized, thereby forming an interlayer insulating film 303 consisting of a polyimide (refer to FIG. 1B). Subsequently, an upper wiring layer 304 is formed on the interlayer insulating film 303 and a polyimide film is formed to cover the upper wiring layer 304 in accordance with the same method as that employed to form the interlayer insulating film 303 to thereby constitute an insulating protective film 311 (refer to FIG. 1C).
As mentioned above, both the interlayer insulating film 303 and the insulating protective film 311 are formed by spin-coating polyamide acid solution on the associated surface. Therefore, the concaves of the uneven surface of the substrate due to the lower wiring layer 302 and the upper wiring layer 304 are filled with the polyamide acid to flatten the surfaces of the interlayer insulating film 303 and the insulating protective film 311.
However, after coating the polyamide acid by using the spin coater, the organic insulating film is subjected to heat treatment at temperatures of 200 to 300xc2x0 C. for several tens minutes to one hour. In a case where the lower wiring layer and the upper wiring layer consist of aluminum, it has been found that problems associated with the heat treatment appears as hillock of aluminum. Moreover, the thermal reflow performed at temperatures over 200xc2x0 C. imparts thermal stress to the underlying aluminum.
Accordingly, it is an object of the present invention to provide a flattening method for easily flattening an uneven surface of a substrate having an underlying film formed thereon by forming an organic film on the substrate and subjecting the organic film to a heat treatment at temperatures of 200xc2x0 C. or less to cause reflow of the organic film, and a method for manufacturing a liquid crystal display device using the flattening method.
A method for flattening an organic film according to the present invention comprises the steps of forming an organic film on a surface of an insulating substrate and infiltrating an organic solvent into the organic film, and causing dissolution of the organic film to flatten the organic film.
The method for flattening an organic film according to the present invention is constructed such that the organic film is an organic material capable of dissolving into the organic solvent, the organic material is any one of acrylic, polyimide and polyacrylicamide, and the organic solvent includes at least one selecting from the group consisting of Alcohols, Ethers, Esters, Ketones, Glycols, Alkylene, glycols, Alkoxy alcohols and Glycol ethers.
The method for flattening an organic film according to the present invention is further constructed such that the organic film is subjected to heat treatment at temperatures of 100 to 180xc2x0 C. to evaporate the organic solvent included in the organic film after the step for causing dissolution of the organic film to flatten the organic film.
As mentioned above, after coating a surface of the insulating substrate with the organic film, the organic solvent is infiltrated into the organic film to cause the dissolution of the organic film, thus flattening the organic film. Furthermore, after flattening the organic film, the heat treatment for evaporating the organic solvent included in the organic film is performed at relatively low temperatures, i.e., temperatures of 100 to 180xc2x0 C. and therefore, the thermal stress applied to a material wiring layer as a lower layer covered with the organic film is reduced.
A method for manufacturing a liquid crystal display device according to the present invention is constructed as follows. That is, the liquid crystal display device is formed by disposing a TFT substrate and an opposing substrate facing each other and interposing a liquid crystal the TFT substrate and the opposing substrate. In this case, the TFT substrate is formed by the steps of forming a gate line and a gate electrode on a first substrate, forming a gate insulating film covering the gate line and the gate electrode on the first substrate, forming a semiconductor film on the gate insulating film and then, forming a source/drain electrode on the semiconductor film to connect the source/drain electrode to the semiconductor film, forming a protective film covering the semiconductor film and the source/drain electrode on the gate insulating film and forming a flattened film on the protective film, in which the step for forming a protective film is performed such that an organic film is coated on the protective film and an organic solvent is infiltrated into the organic film to cause dissolution of the organic film.
As described above, according to the method for forming the liquid crystal display device, the flattened film on the protective film of the TFT substrate is formed by infiltrating the organic solvent into the organic film to cause reflow of the organic film to flatten the organic film after coating the organic film on the protective film, and further, the organic film is subjected to heat treatment at relatively low temperatures, i.e., temperatures of 200xc2x0 C. or less, thereby reducing thermal stress applied to wiring layers covered by the organic film.